Broadband coupling between helix and coaxial line



C. B. MAYER Aug. 21, 1962 BROADBAND COUPLING BETWEEN HELIX AND COAXIAL LINE Filed April 13, 1959 lly i; I

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ilhited fi trates atent @hice aosaess BROADBAND COUPLING BETWEEN HELH AND CGAXIAL LINE Charles B. Mayer, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 13, 1959, Ser. No. 806,052 7 Claims. (Cl. 3153.5)

This invention relates to a coupling device for facilitating transfer of electromagnetic energy between an interaction helix and other wave propagating structures.

For certain applications, electron tubes including a hybrid extended interaction structure have been found to possess certain inherent advantages over the other more conventional tubes such as tetrodes, klystrons or travelling wave tubes in that either the gainbandwidth product is greater or the physical dimensions of the tube are relatively smaller. These tubes generally comprise a hybrid construction of a conventional tetrode with cathode, control grid and accelerating grid for density modulating an electron beam and a slow wave helical line in a coaxial cavity for converting the kinetic energy of the modulated electron beam to electromagnetic wave energy.

In such tubes, the helix is resonant at frequencies at which its length is a multiple number of half wavelengths of electromagnetic wave energy involved. Thus, for purposes wherein a constant response is desired, such a property restricts the useful bandwidth of the tube since such resonant frequencies may fall within the band of desired operating frequencies. Self-sustained oscillations may occur at said resonances with a consequent loss of output power as when the tube is used for amplification. However, suppression of oscillations at such frequencies and at submultiples or multiples thereof results in improved bandwidth response without loss of such power.

It is accordingly a principal object of my invention to accommodate a wider band of frequencies for amplification in extended interaction electron tubes incorporating slow wave helices, than heretofore achieved.

It is another object of my invention to facilitat efficient and effective transfer of electromagnetic wave energy between a helical conductor and other circuit components over a relatively broad band of frequencies.

It is still another object of my invention to utilize novel couplers to effectively resonate an interaction helix structure.

In accordance with my invention the output end of a helix near the collector of an extended interaction electron tube is coupled to an output line of a coaxial guide type by a second helix wound about the interaction helix and in an opposite sense to the interaction helix. In accordance with a feature of my invention, the helix coupler is connected at respective ends to the output line on the one hand and to a wall of the coaxial cavity on the other hand. In the construction of the coupler, the number of turns and the pitch of the turns of the coupler may be controlled to effect and achieve proper impedance matching between the interaction helix and the output line to efficiently transfer the desired amplified signal to the load. The frequency band over which the coupler is operable may be controlled by the degree of coupling between helices as determined by the spacing between the same and between the coupler helix and the cavity end wall.

As a further feature of my invention, another helical coupler of similarly novel construction is disposed about the interaction helix at another location and is useful in an amplifying function of the tube to load the helix at harmonic frequencies to the amplified frequency or at undesired resonances of the helix to suppress oscillations. Such further helical coupler is also wound about the in- The interhelix spacing and spacing between coupler helix and coaxial cavity end wall are controlled to appropriately load the interaction helix. The ends of such second coupler helix ar connected to an external line terminated in a characteristic impedance at the frequencies desired to be suppressed and to the outer wall of the cavity, respectively.

In an oscillating function of the tube, and in accordance with another feature of my invention, the other helical coupler may be coupled to a line connected to a lossless reactance which is controllable to achieve the proper shift in phase of the energy reflected back to the interaction helix in relation to the modulated electron beam to produce sufiicient regeneration on the helix to achieve oscillation in the tube.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the drawing in which:

FIG. 1 is a side elevation in cross section illustrating one embodiment of my invention as applied to an extended interaction tetrode electron tube,

FIG. 2 is an end elevation taken along section 22 in FIG. 1 illustrating my invention in another View,

FIG. 3 is a schematic representation illustrating the applicability of the couplers of the invention in a broad band amplifier incorporating an interaction helix, and

FIG. 4 is a schematic representation illustrating the applicability of the couplers of the invention in an oscillator incorporating an interaction helix.

Referring now to FIG. 1 of the drawing for a detailed description of the invention, It} designates generally the entire tube structure embodying the invention, 12 designates an electron beam producing section generally, 14 represents an interaction cavity section of the tube, and 16 represents generally the electron beam collecting section. The electron beam producing section 12 of the tube 16 is generally of disk seal tube construction and includes as essential elements thereof an electron emitting cathode 18 heated by a coil heater 20, a control grid 22 and an accelerating grid 24. The cathode 18 is mounted at the end of a tubular supporting member 26 in which also is mounted the heater 20 and electrical energy is supplied to the heater through a rigid conductive connection 28 extending through a portion of the suitable ceramic insulator 30 within the tube 26. The wire 28 is connected to a thimble-shaped contact terminal 32 embedded in the insulator 34 The other end of cathode heater 20 is preferably directly connected to the tubular member 26. The end of tubular member 26 near cathode 18 is preferably re-entrant as shown at 34 to confine the heater 20 therein and cathode 18 is preferably welded to the end of the tubular support 26 as shown in the drawing. Suitable electric connections from an external electrical source may be made to members 26 and 32 to energize the heater.

A control grid supporting member 36 is mounted concentrically about the tubular member 26 and spaced therefrom in insulating relation thereto by a suitable ceramic insulator 38 which is preferably bonded to the tubular member 26 and to the tubular member 36. The control grid 22 is formed in the face of a cup-shaped member 4t) telescopically interfitting over the end of the support member 36. The accelerating anode 24 is formed at one end of a tubular member 42 which is mounted in an opening of an end wall 44 of cavity 14.

For supporting the electron beam producing section 12 in insulated relationship with respect to the cavity section 14, the grid supporting member 36 is folded back to form a tubular section 46 terminating in an outwardly directed flange 48. An annular insulating member 50 is disposed between the flange 48 and an inwardly directed flange 52 emanating from a tubular member 53 and is bonded to each of these flanges. The flange 52 terminates in an axial, cylindrical portion fitting in and contacting an interior wall of the end of cavity 14. For providing strength and rigidity to the respective flanged members establishing this support, a pair of further annular insulating members 54 and 56 are bonded or otherwise suitably secured to the portions of the flanges 48 and 52 on the sides thereof remote from the insulating annular member 50.

In a well known manner, an electron beam. between cathode 18 and collector section 16 is produced by heating cathode 18 to a temperature of copius thermionic emission by passing sufiicient current through heater 20 and applying a positive potential to a collector 59 in section 16. The intensity of the beam may be modulated by the value of potential applied to control grid 22 with respect to the cathode. For increasing the velocity of the beam after modulation, a suitable high positive potential with respect to cathode 18, may be applied to accelerating anode 24.

The electron beam produced by section 12 traverses the cavity section 14 which comprises a tubular cylindrical casing member 49 and which is closed at one end by the wall 44 and at the other end by the wall 57. Each of these walls are apertured to accommodate the flow of electrons in this beam. Tubular member 42 is provided with a shoulder 55 and the aperture 45 is provided with a shoulder 47 for the purpose of accommodating and se curing a helical interaction coil 58 preferably made of refractory metal wire with a highly conductive plating. The electron beam passes through the helix and interacts with the electromagnetic field about it in a known manner wherein some of the kinetic energy of the electrons in the beam is transferred to wave energy of the electromagnetic field. For collecting the electron beam in collector section 16, collector electrode 59 has applied thereto a potential positive with respect to cathode 18 from a source not shown.

The collector 59 is supported in position by a cylindrical metal member 60 having an outwardly directed flange 61 which is bonded to one side of an annular ceramic insulator 62. Bonded to the other side of ceramic insulator 62 is a flange 64 of an annular supporting brace 66 having a longitudinal portion secured to the inner end portion of tubular member 52. For purposes of imparting strength and rigidity to the supporting structure, suitable annular ceramic insulating members 68 and 70 are bonded to the sides of flanges 61 and 64 remote from the insulator 62.

In a particular embodiment of my invention suitable for operation at a center frequency of approximately 300 megacycles per second and with a beam potential of approximately 1000 volts, the cavity section 14 is approximately 1.5 inches in internal diameter, the wire of coil 58 is approximately .025 inch in diameter and the helix is approximately .440 inch in diameter and 21 turns per inch of axial length.

For suppressing oscillations of the helix at frequencies harmonic to signal frequencies when operating the tube as an amplifier, a coaxial transmission line 72 is provided for coupling the helix to external means for absorbing and suppressing undesirable energy waves. For removing electromagnetic wave energy from the tube, a coaxial transmission line 74, terminated externally in a characteristic impedance at the frequencies to be amplified, is provided.

The outer conductor of coaxial line 72 is rigidly secured to a flanged bracket 76, the flange of which is provided with apertures accommodating screws 78 and 80 passing through a block 82 and threadedly engaging the tubular member 49. The inner conductor of the coaxial line 72 extends through an aperture 84 in the tubular member 49 and terminates in an enlarged resilient fingered portion 86, shown more clearly in FIG. 2 of the drawings.

To effectively couple line 72 to helix 58, a capacitor designated generally at 89 and a coupler designated generally at 90, are provided. Capacitor 89 includes a hatshaped member 91 of ceramic, insulating material with a brim or flange 92 sealed to an eyelet 93 in the cavity 49. To provide effective conductive capacitor plates 94 and 95, on opposite sides of the hat 91, it is metallized on these opposite sides as by a known titanium hydride or other process. Plate 94 which is within the hat, may be engaged by the resilient fingered portion 86 for contact with the inner conductor of line 72 and outer plate 95 has a cap 96 brazed thereon and to which in turn may be brazed a lead wire for connection to an end of coupler 90. Thus, the capacitor 89 is formed integral with the tube and is mounted therein.

According to my invention, coupling device comprises a helical winding of highly conductive wire wound about the helix 58 in a sense opposite to that of helix 58. The helix coupler 90 is connected at one end to a conductive cap 96 which is in engagement with outer thimble member as shown in FIG. 2. At the other end, helix 90 is shorted to the interior wall of tubular member 49. This coupler effects a maximum energy transfer from. the interaction helix over a band of frequencies harmonic to the desired frequencies and suppresses the unwanted oscillations and the interaction which would otherwise occur in this frequency range.

As another feature of my invention, for improving the performance of the coupling circuit, the capacitance of capacitor 89 is such as to produce a capacitive reactance substantially equal to the inductive reactance of coupler 90 at a frequency in this band. Thus, a circuit which is series resonant at said frequency is formed in the coupling line for facilitating maximum energy transfer from interaction helix 58 and a characteristic impedance termination connected to line 72. This is shown in FIG. 3, wherein the tube 10, including couplers 90 and 98, is shown schematically, and the characteristic impedance 73 is coupled to coupler 90.

Energy is extracted from the helix 58 within tubular member 49 by a coupling device 98 constructed in accordance with features of my invention wherein the coupling device 98 comprises a helix wound about helix 58 in a sense opposite to that of helix 58. The ends of helix '98 are connected, respectively, to an inner wall of tubular cavity member 49 and to a conductive cap 100 on a capacitor 102 formed in an identical manner to that of capacitor 89. An output line 74 has an inner conductor thereof connected to the inner of the thimbleshaped capacitor members of capacitor 102.

In accordance with the particular construction of one embodiment of my invention, for operation in the 200 to 400 megacycles per second frequency band, the helix 98 comprises 1% turns, is spaced from helix 58 approximately 30 mils and has a pitch between successive turns of approximately 60 mils. It is also preferably spaced approximately mils from the end wall 57. The capacitor 102 formed in a manner identical to capacitor 89 is series resonant with the inductance of coupler 98 for maximum energy transfer.

1 The couplers '90 and 98 are well adapted in cooperation with other components for resonating and tuning the helix 58 of tube 10. As shown schematically in FIG. 4 of the drawings, coupler 98 is coupled through capacitor 102 and line 74 to a load designated 104 and coupler 90 is coupled through capacitor 89 to a variable lossless reactance 106. To achieve proper conditions for resonance, coupler '98 is mismatched in its coupling to helix 58 over a relatively broad frequency band whereby a portion of the energy at these frequencies travelling along the helix from the cathode to collector end of the tube 10, is reflected at the collector end and travels back toward the cathode end. At the cathode end of the helix 58', coupler 98 is coupled to helix 58 to match the surge impedance of the helix 58 over a relatively broad band of frequencies. The energy reflected back along the helix 58, travels along line 72 to the reactance 186 where it is again reflected. The phase of such reflected energy may be controlled and shifted so that it appears at the cathode end of helix 58 in the proper phase in relation to electron bunches in the cathode to anode electron beam to extract energy from the beam. Thus, the action of the system is regenerative and oscillations may be produced. The present couplers are admirably suited in obtaining the proper degree of coupling merely by coordinating the spacing between the couplers and helix and between couplers and cavity end Walls.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. 1, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A broad band coupling device operable over a predetermined band of frequencies comprising a conductive cavity and an interaction helix extending therethrough, a helical winding extending about said helix in uniform spaced relationship thereto, a capacitor, one end of said winding being connected directly to an inner wall of said cavity and the other end of said winding being connected to an external line through said capacitor, said capacitor forming with the inductance of said winding a series resonant circuit at a frequency in said predetermined frequency band.

2. A broad band coupling device operable over a predetermined band of frequencies comprising a conductive cavity and an electron tube wave interaction helix contained in said resonant cavity, a helical winding extending about said helix in uniform spaced relationship thereto and wound in a sense opposite to that of said helix, one end of said winding being connected to an internal wall of said cavity and the other end of said winding being connected through a capacitor to one conductor of said line, the inductance of said winding and the capacitance of said condenser forming a series resonant circuit at a frequency within said predetermined band.

3. A broad band coupling device operable over a predetermined band of frequencies comprising a circular cylindrical conductive cavity and an interaction helix coaxially disposed in said cavity, a helical winding disposed in uniform spaced relationship about said helix and being wound in a sense reverse to that of said helix, one end of the said Winding extending radially to the wall of said cavity and being directly connected thereto, an output line, the other end of said winding being coupled through a capacitor to a conductor of said output line, the capacitance of said capacitor and the inductance of said helical winding being series resonant at a frequency within said predetermined band.

4. A broad band coupling device according to claim 3 wherein said helical winding comprises one complete turn.

5. An apparatus comprising an enclosure defining a hollow cavity, an opening in each of a pair of opposite ends of said cavity and an interaction helical winding in said cavity in axial alignment with said openings, means for projecting a beam of density modulated electrons through said winding, a first coupler including a helical winding about said interaction helical winding in a sense reverse to that of said interaction Winding near one end thereof, one end of said first coupler being connected to a Wall of said enclosure and the other end being coupled to an impedance equal to the characteristic impedance of said interaction winding, a second coupler including a helical Winding about said interaction helical winding in a sense reverse to that of said interaction winding near the other end thereof and being matched for maximum energy transfer between the interaction winding and the second coupler, one end of said second coupler being connected to a'wall of said enclosure and the other end being coupled to a load circuit.

6. An apparatus comprising an enclosure defining a hollow cavity, an opening in each of a pair of opposite ends of said cavity and an interaction helical Winding in said cavity in axial alignment with said openings, means for projecting a beam of density modulated electrons through said winding, a first coupler including a helical winding about said interaction helical winding in a sense reverse to that of said interaction winding near one end thereof, one end of said first coupler being connected to a Wall of said enclosure and the other end of said coupler being terminated in a lossless variable reactance, a second coupler including a helical winding about said interaction winding in a sense reverse to that of said interaction winding near the other end thereof and being matched for maximum electromagnetic wave energy transfer between the interaction winding and the second coupler, one end of said second coupler being connected to a wall of said enclosure and the other end being coupled to a load circuit.

7. A broad band coupling device operable over a predetermined band of frequencies comprising a conductive cavity having a wall opening and an interaction helix extending thereacross, a helical winding extending about a portion of said helix in spaced relationship thereto, a capacitor including an insulator having a hollow cylindrical portion and an integral enlarged portion sealed in said wall opening, the hollow of said cylindrical portion and the outer periphery thereof being metallized to form plates of a capacitor, one end of said helical winding being connected to an inner wall of said cavity and the other end of said winding being connected to the exterior plate of said capacitor.

References Cited in the tile of this patent UNITED STATES PATENTS 2,811,673 Kompfner Oct. 29, 1957 2,828,440 Dodds et al. Mar. 25, 1958 2,867,744 Kompfner Jan. 6, 1959' 2,885,593 Cook May 5, 1959 2,894,168 Wing et al. July 7, 1959 2,921,224 Tien Jan. 12, 1960 

